Now, the Boston bombings and the immigrant brothers accused of carrying them out cast a shadow over a hearing yesterday on Capitol Hill. The Senate Judiciary Committee was spending a second day scrutinizing a new bipartisan immigration bill. The measure offers a path to citizenship for millions of immigrants who are now here illegally. It also tries to tighten security along the Mexican border. Some of the bill's opponents brought up the Boston suspects and this had backers of the bill on the defensive.

NPR's David Welna reports.

DAVID WELNA, BYLINE: The 844 page immigration overhaul came out after the Boston bombings. Still, that was before anyone realized the apparent perpetrators were a pair of immigrant brothers who were granted political asylum a decade ago. Once that was known, critics started linking them to the new legislation. As yesterday's hearing on that bill got underway, Judiciary Committee Chairman Patrick Leahy preemptively issued a stern warning.

SENATOR PATRICK LEAHY: Let no one to be so cruel as to try to use the heinous acts of these two young men last week to derail the dreams and futures of millions of hardworking people. The bill before us would serve to strengthen our national security by allowing us to focus our border security and enforcement efforts against those who do us harm.

WELNA: It was the panel's top Republican, Chuck Grassley of Iowa, who first raised questions about the Tsarnaev brothers at the bill's initial hearing last Friday. Yesterday, Grassley compared doing that to raising questions about last week's fertilizer plant explosion.

SENATOR CHUCK GRASSLEY: I don't hear any criticism of people when there's 14 people killed in West, Texas and demanding taking advantage of that tragedy to warn about more government action to make sure that fertilizer factories are safe.

WELNA: Grassley then got into a heated exchange with one of the bill's cosponsors, New York Democrat Charles Schumer, who declared the American people did not want delays or impediments to the immigration bill.

SENATOR CHARLES SCHUMER: I say that particularly those who are pointing to what happened, the terrible tragedy in Boston, as a - I would say excuse for not doing the bill or delaying it many months or years.

GRASSLEY: I never said that.

SCHUMER: I didn't say you did.

GRASSLEY: I never said that.

SCHUMER: I didn't say you did, sir.

WELNA: In fact, senators generally steered clear of linking the Boston bombers to the immigration bill. But that was not the case with some of the witnesses. Mark Krikorian of the Center for Immigration Studies said the bombing illustrated problems in the current immigration system.

MARK KRIKORIAN: Why were the Tsarnaevs given visas to come to the United States to begin with? This is a question nobody seems to have answered. Why were they given asylum since they had passports from Kyrgyzstan? And especially why were they given asylum since the parents have moved back to Russia, the country they supposedly were fleeing?

WELNA: And Kansas Attorney General Kris Kobach pointed out that the older Tsarnaev brother who was killed last week had likely gotten more screening from the FBI than most unauthorized immigrants would get as they seek citizenship.

KRIS KOBACH: Most of these aliens are not going to have personal interviews. Tsarnaev had at least two background checks and he had a personal interview with the FBI. Yet they were still unable to conclude that he might have terrorist intentions and should be barred from the country, and that was far more scrutiny than these aliens are going to have.

WELNA: And that's precisely why this immigration legislation is needed, said South Carolina Republican and bill cosponsor Lindsey Graham. It gets people into the system.

SENATOR LINDSEY GRAHAM: All the hijackers on 9/11 were visa overstays. And if Boston tells us anything, we need to be aware of who's living among us, whether native born or come in on a visa and become a citizen - we can be threatened by our own people.

WELNA: One thing all did agree on was a request from Chairman Leahy to honor Boston's victims.